The present invention relates to a capacitive position transducer for detecting position changes, particularly, to a capacitive position transducer used for producing position feedback signals in a control system or in a recorder or for detecting relatively long distance position changes in a device.
A well-know capacitive position transducer comprising a pair of differential capacitors has two disadvantages: first, the shapes of the electrodes must meet stringent requirements of high accuracy; and second, because the variation of the distance between electrodes affects the output signal significantly, it is difficult to obtain high accuracy. In another kind of capacitive position transducer (see Japanese Patent Application No. 112769), arbitrary nonlinearity is obtained by an additional voltage divider. This second capacitive position transducer has the following shortcoming: the voltage divider with either resistors or capacitors is influenced by parasite capacitors and, consequently, it is necessary to adjust an adjustable device to implement an exact voltage dividing. Other shortcomings are the same as in the differential capacitive position transducer, namely, sensitivity to the variation of the shapes of, and distance between, the electrodes. As a result, it is difficult to attain an accurate detection of position changes with such known transducers.
A linear motor, particularly, a brushless linear motor (a linear motor with an electronic commutator), is the most desirable servomotor in electronic servorecorder.
However, a brushless linear motor needs a commutating signal, and the use of a commutator will increase the complexity of the device significantly. The increased complexity forms the main obstacle to the application of brushless linear motors in recorders.
An object of the present invention is to provide a capacitive position transducer in which the above-mentioned drawbacks are overcome.
It is another object of the present invention to improve the accuracy of the detection of such a capacitive position transducer without enhanced requirements for high machining precision during the manufacturing of the transducer and at the same time to achieve a high accuracy of nonlinear output characteristics. In addition, it is also intended to improve the reliability and environmental immunity of the transducer to attain a high cost-effectiveness.
Yet another object of the present invention is to provide a new technique for producing commutating signals by transducer itself, which makes the use of a brushless linear motor more economic in recorders or other control systems.